Tapering block (5)
I believe the answer is:
wedge
'tapering block' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for wedge that I've seen before include "Tapering object pushed into a gap - golf club" , "Triangular, thick piece" , "Significant amount of money (informal)" , "Tapering block" , "Piece of wood thick at one end, thin at other" .)